Volkswagen's chief executive has said sorry after US regulators found some of its cars disguised pollution levels.
"I personally am deeply sorry that we have broken the trust of our customers and the public," Martin Winterkorn said.
He has launched an investigation into the device that allowed VW cars to emit less during tests than they would while driving normally.
The company's shares were down 19% in Frankfurt by lunchtime.
VW has stopped selling the relevant diesel models in the US, where diesel cars account for about a quarter of sales.
The German carmaker was ordered to recall half a million cars on Friday.
The Environmental Protection Agency (EPA) found the "defeat device" in diesel cars including the Audi A3, VW Jetta, Beetle, Golf and Passat models.
In addition to paying for the recall, VW faces fines that could add up to billions of dollars. There may also be criminal charges for VW executives.